The manufacture of high density, narrow line width integrated circuits requires the use of high resolution lithography equipment. Because of current limitations in the resolution capability of optical lithography equipment, other wafer topography defining processes, such as those employing extremely short wavelengths, pattern forming beams (e.g. X-rays have been developed.
A critical component in the successful utilization of X-ray lithography equipment is the mask through which the X-ray beam defines the topography of the processed wafer. A typical X-ray mask employs an X-ray absorption pattern, usually comprised of a heavy metal, such as gold or tungsten, that is formed on an underlying support membrane. Not only must this membrane be capable of providing a support base for the absorption metal pattern, but it must be highly transparent to the wafer-processing write X-ray beam and to visible light. In this regard, it is normally required that the support membrane have a X-ray transmissivity of at least 50%, that it have a high transmissivity to visible light, particularly laser light, and the physical size of the mask itself must correspond to the size of the underlying (e.g. silicon) substrate to be patterned. Moreover, in terms of its structural parameters the mask, which is planar, must be dimensionally stable and of a uniform thickness with minimal defects.
Materials which have been commonly employed in the construction of X-ray mask for semiconductor lithography processing include carbon and carbide/nitride compounds, such as silicon nitride, boron nitride, silicon carbide, boron carbide, as well as materials such as silundum and silicon, and materials which utilize polymer films. Unfortunately, each material is not without its own problems. For example, silicon nitride, although enjoying thermal and dimensional stability and a high transmissivity to visible light, suffers from low tensile strength. Boron nitride, which is commonly manufactured by chemical deposition, suffers from surface defects. Silicon itself, even though being a material that is directly applicable to the manufacture of semiconductor integrated circuits, has poor transmissivity to visible light.
The choice of organic materials for use as a support membrane has not be actively pursued since such materials typically suffer from thermal and dimensional stability and have a short useful life.
For an illustration of prior art which discloses the use of carbide, nitride and silicon compounds, such as silicon nitride, boron nitride, silicon carbide, etc. attention may be directed to U.S. Pat. Nos. 3,873,824 and 4,608,326, which particularly address the use of silicon carbide as an X-ray mask support material.
Carbon itself, on the other hand, particularly diamond-like carbon, enjoys high transmissivity to both visible rays and X-rays. Moreover, its thermal, mechanical and chemical stability are excellent and a homogeneous film of carbon can be formed by plasma chemical deposition. Consequently, the properties of a membrane formed of high quality carbon make it a particularly attractive material as an X-ray processing mask.
Literature which discloses the use of carbon as the material of the support membrane includes the U.S. Pat. to Brady et al, No. 4,436,797 and published Japanese patent applications Nos. JP 62-174,378 to Morida Chinso, entitled "Manufacturing Method of a Hardened Carbon Film", published July 31, 1987, and JP 61-324,215 by Susuki Kumi, entitled "X-ray Exposure Mask", published by Feb. 15, 1986.
The '797 Brady et al patent discloses the use of hydrogenated amorphous carbon, in which carbon is deposited in the presence of hydrogen at a substrate temperature of less than 375.degree.. The hydrogen concentration in the resulting film is greater than 1% and the optical bandgap of the manufactured film is greater than 1 electron volt, and preferably 2 electron volts.
In the process described in the Chinso publication, a hardened carbon film is formed by reacting an organic monomer in the presence of hydrogen using a plasma CVD process and results in a film having a internal peel stress of 8.times.10-9.2.times.10-dyne/cm.sup.2. In the Kumi publication, the X-ray exposure mask produced is formed of a diamond-like carbon film and a built up layer structure containing an organic polymer film.
In conventional processes for depositing carbon films, a simple plasma chemical vapor deposition process is employed. Where a silicon substrate is employed as the support for the carbon film, the substrate is subjected to compressive internal stress, which leads to a warping of a carbon film during subsequent mask processing.
More particularly, with reference to the drawings, FIG. 1(a) shows the formation of a carbon film 102 on an underlying silicon substrate 101. Carbon layer 102 is formed on the silicon substrate 101 by plasma chemical vapor deposition, in which an upper electrode is grounded and the silicon substrate is coupled to a negative potential with a radio frequency voltage of 13.56 MHz established between the electrodes. With the introduction of a reaction gas such as CH4 or C2H6, and a reactor pressure on the order of 5-100 m Torr, carbon film 102 is deposited upon the silicon substrate 101. The rate of deposition of the carbon depends upon ambient pressure and RF 25 power within the reactor and is typically on the order of 100 .ANG. per minute.
A change in the conditions of the deposition process will affect the optical properties of the deposited carbon film 102. In particular, when using methane as a reaction gas, the optical bandgap may be varied between 0.5 eV to 2.5 eV by modifying the ambient conditions within the reactor chamber. The optical bandgap depends significantly upon a self-biased formed on the reactor electrode to which the substrate 101 is coupled. As this self-bias is produced, the optical bandgap increases.
As mentioned previously, one of the factors in the quality of the carbon membrane as an X-ray mask support layer is its internal stress. For an optical bandgap in a carbon film greater than 1.5 eV, the compressive stress within the film is greater than 5.times.10 dyne/cm.sup.2. As a consequence, when the based silicon substrate 101 upon which the carbon layer is deposited is etched away, so as to provide a silicon support border around the membrane, the internal compressive stress causes the carbon film 102 to warp as shown at (b) of FIG. 1, thus severely altering the planarity of the carbon support membrane and whereby the quality of the X-ray mask.